The present invention relates to a method and an arrangement for generating steam from black liquor at a digester plant of a chemical pulp mill.
Prior art utilizes a fiberline system with a chip bin, where wood chips or other cellulosic material is steamed and liquid is admixed therein for forming a suspension, after which the suspension is pressurized (this is also referred to as a feed system), fed into a treatment vessel or treatment vessels (which may be an impregnation vessel, a pre-hydrolysis process or other vessels), whereafter follows a digester (this part is also referred to as a cooking system). At present, at least one black liquor stream (typically at a temperature of 120-170° C.) is discharged from the cooking system. The extracted black liquor stream or streams is/are used as a heat source for “pre-heating” white liquor, other black liquor streams and/or other liquid streams being led to the feed and cooking systems. The discharged black liquor stream or streams is/are then led to a pre-evaporation system, i.e. to one or several flash tanks, wherein steam is generated from hot black liquor as it is cooled, typically to a temperature of approximately 100-120° C. After possible further cooling to 90-95° C. the black liquor is led to an evaporator system of the recovery area. The thus generated flash steam can be used at another location in the pulping process. Flash steam can e.g. be used for direct preheating of chips prior to cooking.
The above described flashing process, although being used successfully in conventional continuous digesters, has the disadvantage that the generated steam contains volatile compounds, including sulfur compounds, which are not desirable in wood chip presteaming. Typically, wood chips are steamed at atmospheric or slightly higher pressure so that residual gases are not absorbed into the wood chips, but they are collected and treated. The treatment is typically combustion in the non-condensable gas (NCG) system of the mill. This collection and treatment system becomes especially significant when the used steam contains volatile compounds, including sulfur compounds, which have a disadvantageous impact for the environment, including detrimental odor. Therefore, it is advantageous to use such a heat source that minimizes or eliminates the introduction of volatile compounds into the steaming process. In addition, an explosion risk exists, if concentrated gases entrained in flash steam are introduced into low concentration gases.
U.S. Pat. No. 6,722,130 discloses a system for producing clean steam from black liquor. First the pressure of the black liquor is decreased for producing a second black liquor having a higher concentration and black liquor steam, which is condensed to condensate. This condensate is heated with the first black liquor and flashed for producing clean steam to be used in a chip bin.
U.S. Pat. No. 6,176,971 discloses a system for producing clean steam for use in a chip bin. Substantially clean usable steam is produced from hot waste treatment liquor (e.g. black liquor) by leading the waste liquor to a reboiler and then pressurizing (e.b. by means of an eductor, fan or compressor) this clean steam discharged from the reboiler.
A further process for producing clean steam is disclosed in U.S. Pat. No. 6,306,252 for use in a chip bin, whereby black liquor obtained from a digester is led through a heat exchanger wherein clean process water is heated, after which the pressure of the heated process water is decreased, whereby clean steam is generated.
WO 2007073333 discloses a system and a method for producing steam at a digester plant of a chemical pulp mill. The pressure of hot pressurized black liquor obtained from the digester is decreased in a first stage for producing black liquor steam that is used for chip pretreatment in a second preheating stage. Clean steam for pretreating chips in the first preheating stage is generated by re-heating the black liquor, the pressure of which was decreased prior to the final subsequent pressure decrease, where the increased black liquor steam volume is led into a steam converter for producing clean steam.
Known solutions offer various systems for producing cleaner steam for the heating needs of a digester plant and for improving the energy economy of a chemical pulp mill.